This invention relates generally to video displays for multiple video modes and, more particularly, to x-ray protection for cathode ray tube displays.
Protection against generation of harmful X-radiation from a cathode ray tube (CRT) includes an X-ray protection (XRP) circuit that compares a sense voltage, representative of an ultor voltage, against a reference voltage. Generation of the ultor voltage is disabled when the sense voltage is greater than the reference voltage. Accuracy of the XRP circuit to disable generation of the ultor voltage at a proper level relies on the sense voltage maintaining a predetermined relationship to the ultor voltage. This relationship is influenced by the relationship between beam current and ultor voltage. As indicated by the high voltage versus beam current curves 15 or 16 in FIG. 1, the slope or impedance is steeper at low beam current than at high beam current.
In monitor or CRT display applications the beam current and ultor voltage are maintained below the CRT's isodose curve. The isodose curve defines variations in ultor voltage and corresponding beam current at an anode of the CRT for a relatively constant level of X-radiation by the CRT. The isodose curve is a trip curve in that when beam current and ultor voltage are above the isodose curve the XRP circuit disables generation of the ultor voltage. As observed from FIG. 1, isodose curves 11 and 12 define high voltage VHV in kilovolts (kV) versus beam current (Ib) in microamps for X-radiation levels of 0.5 mR/hr (milliroentgen per hour) and 0.1 mR/hr, respectively. The CRT is operated so that its ultor voltage and corresponding beam current coincide below a particular isodose trip curve to avoid a particular level of X-radiation. Although reduced light output has, in the past, been acceptable in computer monitor applications, in television applications maximum light output is the goal and the high voltage is regulated to operate the CRT as close as possible to its isodose curve and improve the focus at high beam currents.
In a television or monitor a secondary winding, conventionally referred to as an X-ray protection winding, on the high voltage transformer develops a voltage VXRP as the primary of the transformer is driven by a pulse voltage waveform at a particular frequency related or synchronized to the video signal's horizontal scan frequency. The voltage VXRP develops with an amplitude that is proportional to the ultor voltage applied to a CRT's anode. The relationship between the ultor voltage and XRP voltage remains relatively constant over a given range of beam current when the transformer is driven by a pulse at a constant frequency.
Various video signal modes have different horizontal frequencies that require different high voltage generator frequencies at which the transformer is energized. High voltage generators incorporating scan-independent high voltage systems can have variable generating frequencies. The standard definition NTSC signal, high definition ATSC signal, and computer generated SVGA signal have the following respective horizontal frequencies, 15.734 kHz (1H), 33.670 kHz (2.14H), and 37.880 kHz (2.4H). Selection to a higher horizontal frequency signal will require driving the high voltage transformer with a pulse voltage waveform at a higher frequency. For example, in the NTSC broadcast signal mode, the high voltage generator is synchronized to the horizontal scan frequency but operated at 2H or 31.468 kHz, and in the SVGA monitor mode the high voltage generator is locked to the 37.880 kHz (2.4H) video signal frequency.
The high voltage transformer which develops the ultor voltage and voltage VXRP operates with a frequency dependent impedance. As frequency of the voltage energizing the transformer increases the inductive coupling to the secondary winding developing the ultor voltage becomes much more lossy than the inductive coupling to the secondary winding developing the voltage VXRP. Known frequency dependent transformer losses in the inductive couplings between the primary winding and secondary windings may include losses due to inter-winding capacitance and eddy current effects. Energy is dissipated during the charge and discharge of inter-winding capacitance between winding layers of the transformer. At a greater energizing frequency the effects of inter-winding capacitance are more pronounced. Also, at higher frequencies known skin effects occur in which conductors appear to have a higher AC resistance from current crowding at the surface of the conductor. With multiple winding conductors skin effects are more pronounced at greater energizing frequencies. Although these and other types of known transformer losses will vary with transformer construction, the losses will be greater with increases in frequency at which the transformer is energized.
To compensate for the increased loss in inductive coupling producing the ultor voltage and maintain a relatively constant ultor voltage, as frequency increases the pulse voltage driving the primary winding of the transformer is boosted to maintain the ultor voltage relatively constant. Since the inductive coupling to the secondary winding developing the voltage VXRP is not as lossy as that for developing the ultor voltage, voltage VXRP increases as the primary voltage energizing the transformer is increased to maintain the ultor voltage level. As a result, voltage VXRP increases relative to the ultor voltage and cannot be used directly to monitor and determine fault levels in ultor voltage over changes in frequency.